Silicon carbide has long been recognized as an extremely desireable substance for use as a semiconductor material Silicon carbide has a number of characteristics which make it theoretically advantageous for such uses. These include a wide band gap, a high thermal conductivity, a low dielectric constant, a high saturated electron drift velocity, a high breakdown electric field, a low minority carrier lifetime, and a high dissociation temperature. Taken together, these properties indicate that semiconductor devices formed from silicon carbide should be operable at much higher temperatures than devices made from other semiconductors, as well as at higher speeds and higher power levels. Silicon carbide's particular band gap and the associated transitions also makes it an appropriate candidate for producing light emitting diodes (LED's) in the blue portion of the visible spectrum.
More recently, significant progress has been made in the development of commercially viable silicon carbide suitable for electronic devices, including the growth of bulk crystals and the growth of epitaxial layers of both beta and alpha type silicon carbide upon silicon carbide substrates. Some of these techniques are set forth in U.S. Pat. Nos. 4,912,063; 4,912,064; 4,865,685; and 4,866,005, and associated devices of particular quality and usefulness are set forth in other patents; e.g. Pat. Nos. 4,875,083 (MIS-type capacitors); 4,918,497 (blue light emitting diodes); 4,947,218 (p-n junction diodes); and 4,945,394 (bipolar junction transistors).
These successes have resulted in some of the highest quality silicon carbide ever produced by either bulk or epitaxial growth methods. Therefore, they have in turn triggered further investigations and developments, and have raised new problems and challenges of which earlier workers were simply unaware.
In particular, silicon carbide, and specifically silicon carbide epitaxial layers grown by chemical vapor deposition, have a carrier concentration of residual nitrogen that generally always is, and has always been, at least 5.times.10.sup.16 atoms per cubic centimeter (cm.sup.-3) ("5E16"). Conventional wisdom holds that this residual nitrogen is a consequence of nitrogen present in the source and carrier gases used during chemical vapor deposition. The result is that silicon carbide epitaxial layers will always have donor atoms present to at least this extent. Although this may not always present a problem when the resulting silicon carbide is to be n-type, it raises significant difficulties when p-type silicon carbide is desired, and also causes problems in controlling the donor population of n-type layers. Because of residual nitrogen, such p-type epitaxial layers will always be "compensated" with a nitrogen donor to at least the extent of 5.times.10.sup.16 cm.sup.-3. As is known to those familiar with semiconductor devices and technology, such compensation can limit the usefulness or application of devices produced using such materials.
For example, in rectifying diodes, a higher population of donor (n-type) carriers in the p-type material decreases the carrier mobility of the resulting rectifier. Stated differently, the development of rectifying diodes with desireable or necessary higher reverse breakdown voltages requires a minimization of donor carriers in certain layers. In light emitting diodes ("LED's), where compensation is desired in at least one layer, the level of compensation is always critical, and variations in the amount of nitrogen present will raise serious difficulties.
In conventional semiconductor techniques, however, the reasons for the presence of nitrogen as an unwanted residual donor have not been recognized. This lack of recognition of the problem raised by nitrogen with respect to silicon carbide probably results from the fact that nitrogen does not act as a donor in silicon, the most widely used semiconductor material. Thus silicon, the most widely used semiconductor material the presence of 5E16 cm.sup.-3 of nitrogen in silicon does not present the problem that such a concentration presents in silicon carbide.
Furthermore, chemical vapor deposition growth of silicon carbide typically takes place at temperatures much higher than the temperatures at which corresponding CVD growth of silicon takes place. For example, CVD of silicon takes place at temperatures of no more than about 1200.degree. C., while that of silicon carbide preferably takes place at least about 1400.degree. C. or higher. At higher temperatures, there exists a greater probability that the problem of impurities will be exacerbated. In CVD growth of silicon carbide, however, higher temperatures promote fewer defects in the resulting crystals. Thus, higher quality LED's are preferably formed from epitaxial layers grown at temperatures of at least 1500.degree. C., and desirably at even higher temperatures, but conventional CVD systems and susceptors are only suitable at temperatures of about 1200.degree. C. or less.
For example, most susceptors used for chemical vapor deposition are commonly formed of graphite and then coated with a layer of silicon carbide. The graphite provides a material with an appropriate resistivity for being inductively heated by electromagnetic energy within the medium portion of the radio frequency ("RF") range; usually the kilohertz (kHz) range. The silicon carbide coating provides an appropriately inert material upon which substrates can be placed during higher temperature operations.
In contrast to the conventional wisdom as to the root of the nitrogen contamination problem, the present inventors have discovered that much, and very likely all, of the undesired contamination in silicon carbide is a result of nitrogen gas that escapes from the susceptors during chemical vapor deposition. Because CVD growth of silicon carbide typically takes place at temperatures well above those necessary for CVD growth of silicon, such "out gases" may not be generated during silicon growth, and the problems they raise have accordingly remained unobserved prior to the more recent advances in silicon carbide technology described earlier.
It has thus only recently been observed that at the high temperatures required for CVD growth of silicon carbide, the silicon carbide coatings on most graphite susceptors begin to develop minute mechanical failures, often exhibited as cracks or pinholes. Because graphite is porous and absorbent for many gases, gases trapped in the graphite before the susceptor was formed escape through these cracks and pinholes and contaminate the silicon carbide epitaxial layers grown using such susceptors. For example, in most susceptor manufacturing processes, nitrogen is introduced at some point during the coating process. In other cases it appears that susceptors with cracks or pinholes will absorb atmospheric nitrogen every time a CVD growth cycle is completed and the growth chamber opened.
Thus, the relatively high residual n-type carrier concentration of epitaxial layers of silicon carbide grown by chemical vapor deposition has remained a problem for which conventional wisdom and techniques have failed to provide a solution
Accordingly, it is an object of the present invention to produce silicon carbide in which the carrier concentration of nitrogen is less than 2.times.10.sup.16 cm.sup.-3, and to provide a method and associated apparatus for producing epitaxial layers of such silicon carbide by chemical vapor deposition.